The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Provision of aircraft with the ability to link to satellite communication networks necessarily entails the use of antenna, which is generally external to an aircraft. Unlike ground-based or maritime craft, however, the need to provide a suitably aerodynamic profile sets limitations on the size and configuration of such antennae that can limit their performance.
One antenna configuration currently in avionic use is a rectangular antenna that lies along or is angled relative to the aircraft's surface (type 1). Such an antenna is steered mechanically to adjust azimuth. Similarly, elevation is adjusted mechanically. Such antennae are commercially available through Panasonic® and through Viasat®. Another antenna configuration currently in avionic use is a fixed antenna that lies along the aircraft's surface, generally having a circular shape that is steered electronically in both azimuth and elevation (type 2). Such antennae are commercially available through Thinkom®, Kymeta®, and Phasor®, for example. Generally, a type 1 antenna has a higher antenna profile (d) than a comparable type 2 antenna, which is undesirable from an aerodynamic standpoint. There are, however, important differences in performance characteristics.
An important factor in the suitability of the performance of such antennas is their performance at different latitudes, as communications satellites are generally placed in equatorial orbits (i.e. 0° latitude). This is largely a function of the antenna gain. Antenna gain can be understood as the power flux of a signal intercepted by the effective aperture (Ae(ε)) in a specified direction. Generally, at a given elevation angle ε, gain (G(ε)) can be calculated using the following formula:G(ε)=η(4πAe(ε)/λ2)
For type 1 antennae, Ae (ε) is effectively the area of the rectangular antenna surface (A1). For type 2 antennae, Ae (ε) is the area of the antenna surface multiplied by the sine of the elevation angle (i.e. A2*sin(c)). As a result, all other factors (e.g., efficiency, frequency, footprint, etc.) being equal, the gain of a type 1 antenna remains constant at different elevation angles while the gain of a type 2 antenna is sharply reduced at low elevation angles (see FIG. 1). Consequently, an antenna of type 1 configuration would be expected to support satellite communication over a broader range of latitudes than an antenna with a type 2 configuration having a similar footprint. Such type 1 antennae, however, have a skew angle issue resulting from beam asymmetry that limits their use at longitudes far from the target satellite (due to interference to neighboring satellites). Antennae having a type 2 configuration have less of a skew angle issue; however, this reduction in interference to neighboring satellites is accompanied by reduced gain at higher latitudes. These effects are shown in FIG. 2A (showing the relationship between gain, attitude, and relative longitude for a type 1 antenna) and FIG. 2B (showing the relationship between gain, latitude, and relative longitude for a type 2 antenna).
An at least partial solution to the skew angle problem experienced with type 1 antennas is to electronically distort or rotate the asymmetric beam produced so that the longer plane of the beam is orthogonal to the arch described by the set of communication satellites. While this can reduce the amount of interference to non-target satellites, such a solution adds to the complexity of the communication system and may not be suitable for harsh operating environments (where mechanical systems can be more reliable). In addition, such a solution does not address the differences in antenna profile. Recently, phased array solutions have been provided but are, to date, prohibitively expensive for many uses. As a result, current technology provides either a wide coverage antenna with an undesirably high profile or a low profile antenna with relatively low coverage.
Thus, there is still a need for an antenna design that supports communication over a wide range of latitudes while minimizing the antenna profile.